So I have very basic knowledge of Hammer editing, I've made some maps in the standard Portal 2 editor and I've did some re texturing, sometimes I've added onto the map, but this is my first time attempting a map from scratch. I decided to go with an underground setting for the puzzle. I had a very basic layout of the map complete, so I decided to try and get an entrance elevator in to really start.

I placed in the 'Test_dome_entrance_lift_01' instance, edited a 'logic_auto' into it, and figured I'd test it to make sure it was running smoothly. Up until this point I've had no trouble compiling the map. Ever since that point, I've been getting this error message:

That message has not left even after I took out the elevator completely. I'm not sure what it's supposed to mean, or if I triggered something else while trying to get my elevator in, but I've not been able to find a solution.

EDIT: So it turns out Hammer just didn't like one of the textures I used in my map? I don't know what caused it, but as soon as I replaced the texture, it was working just fine. Not sure why Hammer would decide it doesn't like the texture, but I'll be sure to stay clear of it from now on.

It's not your fault. The material is just broken (= Valve's texture artist made a mistake there), so you get this error when you use this material. Anyway this should not cause any problems in your map.